Recently, an increasing interest was focused on the use of AuNPs for biomedical applications including imaging, sensing, gene delivery, drug delivery, and protein immobilization. Usually, chemical synthesis of AuNPs usually involves the reduction of Au3+ ions in the presence of an organic ligand as the stabilizer. Various protection groups such as citrate acid, alkylamine, alkylthiol, cetyltrimethylammonium bromide (CTAB), or polymers must be introduced to mantle the shell of the reduced AuNP surfaces to efficiently prevent the overgrowth and aggregation from the as-synthesized AuNPs. Although AuNPs with reasonable stability and dispersity in solutions can be synthesized with this approach, the post-modification of the AuNP surfaces through ligand exchange usually shows low efficiency and is a time-consuming process since the tight formation of the protective ligands may affect the substitution rate of the exchanged ligands.
More recently, a solution phase synthesis of PEG-protected AuNPs by using synchrotron X-ray as a reduction source has been developed. However, the solution-phase synthesis of AuNPs through a high energy bombardment of the Au3+/PEG-polymer precursor under synchrotron X-ray usually produces a complicated free radical reaction and caused a serious cross-linkage and tangle of the polymerization-PEG outside the shell of AuNPs. Due to the formation of a covalently reticular structure of the PEG chains in the surface of AuNPs, further post-modifications of AuNP surfaces through a ligand exchange are very difficult and not feasible. Moreover, the process of solution phase synthesis of AuNPs through reduction from high energy is usually chemically uncontrollable. It produces unwanted chemical residues from Au3+ ions and the stabilizer, which makes further purification of AuNPs a difficult task.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies related to gold nanoparticle synthesis, especially in connection with chemical reductant (i.e., reducing agent)-free gold nanogold particle synthesis.